1. Field of the Invention
The present invention relates to a cellular mobile communication system, and more particularly, to a method and apparatus for generating a feedback signal to support downlink transmission of a User Equipment (UE) by a plurality of Base Stations (BSs) (or Node Bs).
2. Description of the Related Art
From an early stage of providing only voice-oriented services, mobile communication systems have evolved into high-speed and high-quality wireless packet data communication systems, which provide data and multimedia services. Recently, various mobile communication standards, e.g., High Speed Downlink Packet Access (HSDPA) of the 3rd Generation Partnership Project (3GPP), High Speed Uplink Packet Access (HSUPA), Long Term Evolution (LTE), Long Term Evolution Advanced (LTE-A), High Rate Packet Data (HRPD) of the 3GPP2, 802.16 of the Institution of Electrical and Electronics Engineers (IEEE), etc., have been developed to support high-speed and high-quality wireless packet data transmission services.
The LTE system was developed for efficiently supporting high-speed wireless packet data transmission and maximizes wireless system capacity by using various wireless connection techniques. The LTE-A system, i.e., a wireless system advanced from the LTE system, has improved data transmission capability when compared to the LTE system.
Existing 3rd-Generation wireless packet data communication systems, e.g., HSDPA, HSUPA, HRPD, etc., use an Adaptive Modulation and Coding (AMC) scheme and a channel-sensitive scheduling scheme to improve transmission efficiency. When using the AMC scheme, a transmitter may adjust the amount of data to be transmitted according to a channel state. AMC and channel-sensitive scheduling apply suitable modulation and coding at the most efficient time determined based on partial channel information fed back from a receiver.
In an AMC-applied wireless packet data communication system, a transmitter may adjust the amount of transmission data according to a channel state. For example, in poor channel states, the transmitter may reduce the amount of transmission data to adjust a reception error probability to a desired level, and in a good channel states, the transmitter may increase the amount of transmission data to adjust a reception error probability to a desired level and efficiently transmit a large amount of information.
In a wireless packet data transmission system to which channel-sensitive scheduling resource management is applied, the transmitter selectively services a user having a superior channel state among several users, thereby contributing to an increase in the system capacity, as compared to when the transmitter merely allocates a channel to one user and then services the corresponding user. Such an increase in system capacity is referred to as “multi-user diversity gain”.
AMC, when used together with a Multiple Input Multiple Output (MIMO) transmission scheme, may determine the number of spatial layers or a rank for a transmission signal. In this case, the AMC-applied wireless packet data communication system, when determining an optimal data rate, considers a code rate, a modulation scheme, and the number of layers through which the signal is to be transmitted using MIMO.
Basically, a cellular mobile communication system is formed by establishing a plurality of cells in a limited region. In each cell, a Node B equipment provides a mobile communication service to UEs in the cell. When a mobile communication service is independently provided cell-by-cell, a Reference Signal (RS) for channel estimation is transmitted for UEs in each cell to measure a DownLink (DL) channel state for each cell.
In a 3GPP LTE-A system, a UE measures a channel state between a Node B and itself by using a Channel Status Information Reference Signal (CSI-RS) transmitted from the Node B.
However, a conventional feedback technique only considers only a CSI feedback of a single Node B, i.e., a single transmission point, transmitted to a particular UE.
In a cellular mobile communication system, for a UE located at a cell edge, adjacent cells cooperate with each other for data transmission through Cooperative Multi-Point (CoMP) transmission, also referred to as “CoMP” for simplicity. Therefore, in CoMP transmission, considering simultaneous transmissions from various transmission points, a technique for multiple CSI feedbacks is required.